Process for preparing silver catalyst

ABSTRACT

An improved silver catalyst for the oxidation of ethylene with molecular oxygen is made by impregnating a porous support with a silver salt of an acid; subjecting the impregnated support to a multi-stage activation by heating and post impregnating the support with an cesium carbonate or bicarbonate from an anhydrous alcohol solution followed by washing with alcohol solvent and rapid drying to produce a finished catalyst having from 1-6×10 -3  gew of the alkali metal per kg of catalyst.

This is a continuation of application Ser. No. 08/370,550, filed Jan. 9,1995, now U.S. Pat. No. 5,444,034.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a supported silver catalystuseful for the vapor-phase oxidation of ethylene to ethylene oxide. Moreparticularly, the present invention relates to a method of preparing animproved supported silver catalyst post impregnated with cesium.

2. Related Art

The use of supported silver catalysts for the oxidation of ethylene toethylene oxide has been long known in the art. Additionally, over theyears various promoting metals have been added to further enhanceperformance. In particular, the use of alkali metals has been disclosedin various amounts and added by different methods. A very extensivereview of the patent literature is given in G.B. No. 2,043,481A. Suchdisclosures have been somewhat inconsistent in their teachings, as canbe seen by comparing U.S. Pat. No. 2,238,474 in which sodium and lithiumhydroxides were suggested as promoters and potassium and cesium wereshown to be poisons to U.S. Pat. No. 2,671,764 where rubidium and cesiumsulfates were suggested as promoting compounds.

Although alkali metals were suggested generally in the earlierdisclosures, it is also generally true that more recent workers in thefield have considered potassium, rubidium, and cesium as the preferredalkali metals. For example, see the series of patents to Nielson, etal., in which these materials were used in small amounts co-depositedwith the silver--U.S. Pat. Nos. 3,962,136; 4,010,115, and 4,012,425.Still more recently the art has emphasized synergistic combinations ofthe alkali metals. For example, see G.B. No. 2,043,481A cited above andU.S. Pat. Nos. 4,212,772 or 4,226,782. The art teaches, in addition,that the alkali metals may be used to rejuvenate used catalysts, as forexample U.S. Pat. Nos. 4,123,385; 4,033,903; 4,177,169; and 4,186,106.The art teaches that the alkali metals may be deposited either beforethe silver is placed on the support (pre-deposited)--U.S. Pat. No.4,207,210; at the same time the silver is deposited (co-deposited)--U.S.Pat. Nos. 4,066,575 and 4,248,741; or subsequent to deposition of thesilver (post-deposited)--G.B. No. 2,045,636A.

The amount of alkali metal was suggested to be in quite a wide range inthe older art. It was often indicated that large quantities, e.g. up toseveral per cent of an alkali metal could be used. More recently, theart generally has taught that small quantities of alkali metals producethe optimum effect no matter when the silver and the alkali metals weredeposited. Kilty in U.S. Pat. No. 4,207,210 related the optimum amountof alkali metal to the surface area of the support. Exceptions to theabove include patents issued to ICI which teach the use of large amountsof sodium alone (G.B. No. 1,560,480) and potassium in combination withsmaller amounts of rubidium and cesium (U.S. Pat. No. 4,226,782).However, the art generally teaches that the optimum will be found insubstantially lower quantities, perhaps on the order of 50-500 ppm byweight.

It has long been recognized that the method of preparing the catalystaffects its performance. The differing heat "reactivations'" bearwitness to this. Additionally, the impregnating solutions used and theintermediate steps have been found to effect the final catalyst. Forexample, Winnick in commonly assigned U.S. Pat. No. 4,066,575 disclosesan impregnating solution containing silver lactate, lactic acid, bariumacetate, hydrogen peroxide and water. As a class the lactate basedcatalyst are very stable but exhibit low selectivity. The support isimpregnated with the solution and then first activated by heating in aninert atmosphere at 350° C. for and then dried in air at 200° C. for 12hours. The "activated" catalyst is then impregnated with a cesiumsolution and dried in air at 130° C. for 3 hours. The use of the inertatmosphere during the activation step produced a catalyst that was moreselective, but much less stable, i.e., the catalyst lost its activityfairly quickly resulting in shorter run length for a given end of runtemperature.

Armstrong, in commonly assigned U.S. Pat. No. 4,555,501 disclosed usingan impregnating solution containing the silver salt of a neo acid. Theimpregnated support was then "activated" at temperatures of about 200°C. to 600° C. in the presence of air or reduced oxygen atmospheres, thepresence of some oxygen being desirable. The alkali metal, if desired,was then deposited in small quantities (in the range of 260 wppm).

Cesium now appears to be the preferred alkali metal. Various sources ofcesium are catalogued in the prior art, for example, cesium hydroxide,cesium nitrate, cesium chloride, cesium chlorate, cesium bicarbonate,cesium carbonate, and other anion functionalities such as formates,acetates and the like.

U.S. Pat. No. 4,374,260 discloses the coprecipitation of silver andcesium salt, such as the carbonate from a silver carboxylate/aminocomplex.

U.S. Pat. Nos. 4,350,616 and 4,389,338 both show the deposition of CsCO₃on to activated silver catalyst from alcohol solution where the silverwas derived from aqueous silver salt solution.

U.S. Pat. Nos. 4,066,575 and 4,033,903 disclose the preparation ofsilver catalyst from both aqueous and non aqueous salt solutions andsubsequent treatment of the activated silver catalyst with postdeposition of an alkali metal salt such as cesium and anions from loweralcohol and preferable from aqueous solutions. Similarly U.S. Pat. No.4,342,667 discloses the post deposition of cesium on to silver catalystderived from aqueous solutions.

What is most clear is from the prior art relating to post depositionalkali metal is the general interchangeability of aqueous and nonaqueous procedures, i.e. silver catalyst may be prepared by eitheraqueous or non aqueous procedures and the post deposition of alkalimetal may be aqueous or non aqueous. Furthermore, the salt of silver oralkali metal is not specific. Generally the procedures tended to favorthe presence of water.

It has now been found that water at any stage and in any amount isdetrimental to the performance of the final catalyst. Thus, the presentpreparation is characterized as being substantially anhydrous with postdisposition of alkali metal, e.g. cesium.

It is an advantage of the present invention that catalysts ofexceptional stability in use for the preparation of ethylene oxide areproduced, which have high selectivity at high conversions for theethylene oxide process.

SUMMARY OF THE INVENTION

Briefly stated one aspect of the present invention is a catalystprepared by the process of impregnating a porous support having a lowsurface area with a hydrocarbon solution of a silver salt of an organicacid which is substantially free of water and acid and activated byheating in multi-stages in an atmosphere containing less oxygen than airin order to control the combustion of the organic portion of the acidsilver salt. A preferred series of stages is a first temperature in therange of 150° to 200° C. for less than an hour preferably said firstactivation is carried out in an atmosphere containing less than 20 vol %oxygen, heating at a second temperature in the range of from greaterthan 200° C. to 300° C. for less than one hour, heating at a thirdtemperature in the range of 300° C. to 400° C. and finally heating at afourth temperature in the range of from greater than 400° C. to 500° C.for less than one hour. Preferably each of the heating steps is from 1to 30 minutes in duration. The heating atmosphere is controlled toeliminate uncontrolled combustion of the organic portions of the silversalt or solvents by controlling and adjusting the amount of oxygenpresent during said heating. Preferably the atmosphere contains lessthan 3 vol % oxygen of the total atmosphere. The activation produces asupport containing the activated silver.

The catalyst is made by impregnating a porous support, preferably havinga surface area in the range of 0.2 to 2.0 m² /g, with a hydrocarbonsolution of a silver salt of an organic acid. The solution should besubstantially free of both water and acid as this aspect has been shownto be especially beneficial to catalyst performance and hence preferred.The impregnated support is activated by heating wherein, the improvementis the control of the combustion of the organic materials.

In order to modify the activated silver catalyst an alkali metal,preferably cesium, is added.

Another aspect of the invention the activated silver catalyst is asubstantially anhydrous post impregnation with an alkali metal,preferably cesium, to produce a finished catalyst by immersing thesupport in a stationary or circulating stream of the alkali metal in aanhydrous solvent such as ethanol. The optimum amount of alkali metal(s)added will be selected to optimize catalyst performance and will bedependent upon the surface area of the support chosen. That is, morealkali metal will be used on supports which have larger surface areathan on those having relatively small surface area. The term anhydrousas used herein means as free of water as possible, but in any event lessthan 1% water, i.e., substantially anhydrous.

A third aspect of the present invention is the washing of cesiummodified silver catalyst with a lower alcohol. Preferably methanol,ethanol, isopropanol or the like are contacted with and impregnated intothe catalyst, removed from the catalyst and the catalyst dried.

It is believed that the ionic cesium contacting the alumina of thesupport, not covered by silver, is more firmly attached to the polaralumina surface than that on the surface of the silver metal. The washas described preferentially leaches or removes some of the less firmlyheld cesium on the silver while leaving the desirable modifying cesiumon the alumina sites.

The stability achieved by the multi-stage heat activation of silvercatalyst is retained and the selectivity is enhanced by both theanhydrous preparation and the final solvent wash.

The catalyst of the present invention may be employed under oxidizingconditions typical to the art for preparing ethylene oxide by the vaporphase oxidation of ethylene with improved results, especially catalyststability.

The term "inert" as used herein means any gaseous material under theconditions of activation which does not react with silver or any othercomponent of the silver impregnated support. Preferred inert materialsinclude nitrogen, helium and carbon dioxide, but other specificmaterials, including neon, argon, and the like may be used. Thelimitation of oxygen during the activation is of principal concern.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENT

The catalyst of the present invention may contain from 3 to 25 wt %silver on the support. Preferred catalysts prepared in accordance withthis invention contain from 3 up to about 20% by weight of silver,expressed as metal, deposited upon the surface and throughout the poresof a porous refractory support. Silver contents higher than 20% byweight of total catalyst are effective, but result in catalysts whichare unnecessarily expensive. Silver contents, expressed as metal, ofabout 5-13% based on weight of total catalyst are preferred, whilesilver contents of 8-11% are especially preferred.

Catalysts may be made with supports comprising alumina, silica,silica-alumina or combinations thereof. Preferred supports are thosecontaining principally alpha-alumina, particularly those containing upto about 15 wt % silica. Especially preferred supports have a porosityof about 0.1-1.0 cc/g and preferably about 0.2-0.5 cc/g. Preferredsupports also have a relatively low surface area, i.e. about 0.2-2.0 m²/g, preferably 0.4-1.6 m² /g and most preferably 0.5-1.3 m² /g asdetermined by the BET method. See J. A. Chem. Soc. 60, 309-16 (1938).Porosities are determined by the mercury porosimeter method; see Drakeand Ritter, "Ind. Eng. Chem. Anal. Ed.," 17, 787 (1945). Pore and porediameter distributions are determined from the surface area and apparentporosity measurements.

For use in commercial ethylene oxide production applications, thesupports are desirably formed into regularly shaped pellets, spheres,rings, etc. Desirably, the support particles used have "equivalentdiameters" in the range from 3-10 mm and preferably in the range of 4-8mm, which are usually compatible with the internal diameter of the tubesin which the catalyst is placed. "Equivalent diameter" is the diameterof a sphere having the same external surface (i.e. neglecting surfacewithin the pores of the particle) to volume ratio as the supportparticles being employed.

The silver is added to the support by immersion of the support into asolution containing a silver salt of an organic acid which issubstantially free of water and said acid, such as the neo acids(particularly those having at least seven carbon atoms) described inU.S. Pat. No. 4,864,042 which is incorporated herein in its entirety.The silver containing liquid penetrates by absorption, capillary actionand/or vacuum into the pores of the support. A single immersion or aseries of immersions, with or without intermediate drying, may be used,depending in part upon the concentration of the silver salt in thesolution. To obtain catalysts having silver contents within thepreferred range, suitable impregnating solutions will generally containfrom 5-50 wt % silver, expressed as metal, but supplied as silver saltsof organic acids. The exact concentrations employed, of course, willdepend upon, among other factors, the desired silver content, the natureof the support, the viscosity of the liquid, and solubility of the acidsilver salt.

Impregnation of the selected support is achieved in a conventionalmanner. The support material is placed in the silver solution until allof the solution is absorbed by the support. Preferably the quantity ofthe silver solution used to impregnate the porous support is no morethan is necessary to fill the pore volume of the porous support.

The impregnating solution, as already indicated, is characterized as asubstantially water free and acid free organic solution of a silver saltof an organic acid. A hydrocarbon solvent is employed, such as toluene,cyclohexane, xylene, ethyl benzene, cumene or nonene which wouldnormally be water free. Since water is considered to be detrimental tothe preparation of silver catalysts when the method of the invention isused, it should be present in no more than about 0.1 vol % in the silverimpregnating solution, preferably less than about 0.01 vol %.

After the multi-stage activation the support may be impregnated with thealkali metal if desired. It is the purpose of alkali metal to modify thecatalyst and raise selectivity while leaving the improved stabilityintact. When used the amount of the alkali metal on the finishedcatalyst is generally similar to those employed heretofore. Thus theamount deposited will be generally up to about 8×10⁻³ gew/kg catalyst,preferably up to about 7×10⁻³ gew/kg, and particularly about 1 to 6×10⁻³gew/kg (gew=gram equivalent weight). The alkali metals of the periodictable include sodium, lithium, potassium, rubidium and cesium. Forpurposes of the present invention, the latter three alkali metals areparticularly preferred, especially cesium, although sodium and lithiumare not necessarily excluded. The alkali metal salts are dissolved inalcohol solutions, preferably substantially free of water.

In the absence of water in the alcohol solvent, the cesium compound,although poorly soluble, remains evenly distributed through the solventduring evaporization and drying, hence is more evenly distributed overthe silver catalyst. Preferably the alkali metal impregnated catalystsare dried rapidly, e.g. one to two minutes at high temperature, e.g. atleast 100° C. up to 800° C., preferably around 200° C. to 600° C. Thismay be readily achieved by using a moving belt as described herein or byplacing it in a tube and allowing a fast current of hot air to removethe solvent. The drying may be conducted in air or an inert gas.

Catalysts prepared by the procedures above have improved performance,especially stability, for use in the production of ethylene oxide by thevapor phase oxidation of ethylene with molecular oxygen. These usuallyinvolve reaction temperatures of about 150° C. to 400° C., usually about200° C. to 300° C., and reaction pressures in the range of from 0.5 to35 bar. Reactant feed mixtures contain 0.5 to 20% ethylene and 3 to 15%oxygen, with the balance comprising comparatively inert materialsincluding such substances as nitrogen, carbon dioxide, methane, ethane,argon and the like. Only a portion of the ethylene usually is reactedper pass over the catalyst and after separation of the desired ethyleneoxide product and the removal of appropriate purge streams and carbondioxide to prevent uncontrolled build up of inerts and/or by-products,unreacted materials are returned to the oxidation reactor.

In the following examples the catalysts were made from a cumene solutionof a silver salt of neo-decanoic acid as described above. Thecharacteristics of suitable supports are set out below.

The finished catalysts are then tested for activity and selectivity bycrushing and placing 36 grams in a micro reactor consisting of a 1/4inch stainless steel tube which is heated in a salt bath. A feed mixtureof 7% oxygen, 8% CO₂, 15% C₂ H₄, 70% N₂ is passed over the catalyst witha gas space velocity of 5500 hr⁻¹. The pressure is maintained at 300psig (21.69 bar) and the temperature between 200° C. and 300° C. asrequired to maintain an outlet concentration of 1.5 vol % (160 Kg perhour per m³ of catalyst) ethylene oxide. The activity of the catalyst isexpressed as the temperature necessary to maintain the outletconcentration at 1.50 vol % ethylene oxide, the lower the temperature,the more active the catalyst. The selectivity the catalyst is expressedas the mole% of the total ethylene converted to ethylene oxide at theoutlet concentration of 1.50 vol % ethylene. The stability of thecatalyst is measured by the increase in temperature required to maintainthe ethylene oxide at 1.50 vol % divided by 100 hours and is expressedas °C./100 hr.

EXAMPLE 1 (Comparative)

In this example a conventional activation with aqueous Cs deposition wascarried out.

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 5/16 inch cylinders.The support has a surface area of 0.55 m² /g pore volume of 0.3 cc/g,and medium pore diameter of 15μ. A 95 parts of a cumene solution ofsilver neodecanoate, containing 26 wt % silver, was added to 225 partsof the hot support and the mixture was mixed for 20 minutes. Thecatalyst was prepared using one step activation with air at 500° C. andwas impregnated with cesium hydroxide solution in water/alcohol solvent,which was subsequently dried with vacuum. The catalyst was tested underthe condition as outlined above. After 150 hours of reaction time theselectivity to ethylene oxide was 80.9% and the reaction temperature was232° C. The catalyst's performance did not improve with longer reactiontime.

EXAMPLE 2

In this example multi-stage activation was carried out with anhydrous Csdeposition.

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 5/16 inch cylinders.The support has a surface area of 0.55 m² /g, pore volume of 0.3 cc/g,and medium pore diameter of 1.5μ. A 95 parts of a cumene solution ofsilver neodecanoate, containing 26% silver, was added to 225 parts ofthe hot support and the mixture was mixed for 20 minutes. The depositionof silver compound was induced by heating the catalyst to a temperaturethat did not exceed 200° C. in a stream of nitrogen. The residence timeof the catalyst in the heated zone was 2 minutes on a moving belt. Thisstep was repeated at 300° C. and 400° C.

The catalyst was then impregnated for two hours at room temperature inan anhydrous ethanolic solution that contained 525 ppm cesiumbicarbonate. The catalyst was superficially dried by a stream ofnitrogen followed by heating on a moving belt at 200° C. The results ofthe catalyst test are summarized in TABLE 1.

                  TABLE 1                                                         ______________________________________                                        Results of Catalyst Test                                                      Life, hr    Temp. °C.                                                                        Selectivity %, to EO                                    ______________________________________                                        145         237       82.8                                                    500         237       82.8                                                    575         237       83.0                                                    715         236       83.0                                                    ______________________________________                                    

EXAMPLE 3

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 16 inch cylinders.The support has a surface area of 0.55 m² g, pore volume of 0.3 cc/g,and medium pore diameter of 1.5μ. A batch of 225 parts of the supportwas heated to 80° C. and placed under vacuum, 50 mm Hg, then 95 parts ofa cumene solution of silver neodecanoate, containing 26% silver, wasadded and the mixture was mixed for 20 minutes. The deposition of silvercompound was induced by heating the catalyst to a temperature that didnot exceed 200° C. in a stream of nitrogen the residence time of thecatalyst in the heated zone was 2 minutes. This step was repeated at300° C. and at 400° C. The catalyst was then impregnated for two hoursat room temperature in an anhydrous ethanolic solution that contained525 ppm cesium bicarbonate. The catalyst was dried by heating on amoving belt at 200° C. The results of the catalyst test are summarizedin the Table 2.

                  TABLE 2                                                         ______________________________________                                        Results of Catalyst Test                                                      Life, hr    Temp. °C.                                                                        Selectivity %, to EO                                    ______________________________________                                        100         231       82.5                                                    300         230       82.7                                                    450         230       83.0                                                    ______________________________________                                    

EXAMPLE 4

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 5/16 inch cylinders.The support has a surface area of 0.55 m² /g, pore volume of 0.3 cc/g,and medium pore diameter of 1.5μ. 95 parts of a cumene solution ofsilver neodecanoate, containing 26% silver, was added to 225 parts ofthe support and the mixture was mixed for 30 minutes.

The deposition of silver was induced by heating the catalyst up to thedecomposition temperature of the silver salt. This was achieved viaheating in a furnace that has several heating zones in a controlledatmosphere. The catalyst was loaded on a moving belt that entered thefurnace at ambient temperature, which was gradually increased as thecatalyst passed from one zone to the next. The temperature wasincreased, up to 400° C. as the catalyst passed through seven heatingzones. After the heating zones the belt passed through a cooling zonethat gradually cooled the catalyst to a temperature lower than 100° C.The total residence time in the furnace was 22 minutes. The atmosphereof the furnace was controlled to eliminate uncontrolled combustion ofthe organic portion of the silver salt and solvent. This was achievedvia using nitrogen flow in the different heating zones. The amount ofnitrogen was sufficient to inhibit any combustion and to remove anyevolved fumes during the calcination process.

The catalyst was then impregnated for two hours at room temperature inan anhydrous ethanolic solution containing 525 ppm of cesiumbicarbonate. The solution was drained and an equal volume of ethanol wasadded. The catalyst was mixed with the fresh ethanol to remove anyexcess cesium present on its surface. The liquid was drained, and thecatalyst was dried by placing it on a belt that travelled through aheated zone of a furnace in a current of 200° C. air. The residence timein the hot zone, the air flow and the furnace temperature weresufficient to dry all the solvent in the shortest time possible. Thecatalyst was crushed and charged in a tube that was heated by a saltbath. A gas mixture containing 15% ethylene, 7% oxygen, and 78% inert,mainly nitrogen and carbon dioxide, was allowed to flow over thecatalyst under 300 p.s.i. The temperature of the reaction was adjustedin order to obtain ethylene oxide productivity of 160 Kg ethylene oxideper hour per m³ of catalyst. The results of the catalyst test aresummarized in table 3:

                  TABLE 3                                                         ______________________________________                                        Results of Catalyst Test                                                      Life, hr     Temp. °C.                                                                        Selectivity, %                                         ______________________________________                                        350          227       82.8                                                   ______________________________________                                    

EXAMPLE 5

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 5/16 inch cylindersThe support has a surface area of 0.55 m² /g pore volume of 0.3 cc/g,and medium pore diameter of 1.5μ. 95 parts of a cumene solution ofsilver neodecanoate, containing 26% silver, was added to 225 parts ofthe hot support and the mixture was mixed for 30 minutes. The depositionof silver was induced by heating the catalyst to a temperature that didnot exceed 150° C. in a stream of nitrogen. The residence time of thecatalyst in the heated zone was two minutes. This process was repeatedat 200°, 250°, and 300° C. and at 400° C.

The catalyst was then impregnated for two hours at room temperature inan anhydrous ethanolic solution containing 525 ppm of cesiumbicarbonate. The solution was drained and an equal volume of ethanol wasadded. The catalyst was mixed with the fresh ethanol to remove anyexcess cesium present on its surface. The liquid was drained, and thecatalyst was dried by placing it on a belt that travelled through aheated zone of a furnace in a current of 200° C. air. The residence timein the hot zone, the air flow and the furnace temperature weresufficient to dry all the solvent in the shortest time possible.

After drying the catalyst, it was tested in a tube that is heated by asalt bath. A gas mixture containing 15% ethylene, 7% oxygen, and 78%inert, mainly nitrogen and carbon dioxide, was allowed to flow over thecatalyst under 300 p.s.i. The temperature of the reaction was adjustedin order to obtain ethylene oxide productivity of 160 Kg per hour per m³of catalyst. The results of the catalyst test are summarized in table 4:

                  TABLE 4                                                         ______________________________________                                        Life, hr     Temp. °C.                                                                        Selectivity, %                                         ______________________________________                                        200          230       83.7                                                   900          230       83.7                                                   ______________________________________                                    

EXAMPLE 6

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 5/16 inch cylindersThe support has a surface area of 0.55 m² /g pore volume of 0.3 cc/g,and medium pore diameter of 1.5μ. 95 parts of a cumene solution ofsilver neodecanoate, containing 26% silver, was added to 225 parts ofthe hot support and the mixture was mixed for 20 minutes. The depositionof silver was induced by heating the catalyst to a temperature that didnot exceed 150° C. in a stream of a gas mixture containing 2.5% oxygenin nitrogen. The residence time of the catalyst in the heated zone wastwo minutes. This process was repeated at 200° C., 250° C., 300° C. andat 400° C.

The catalyst was then impregnated for two hours at room temperature inan anhydrous ethanolic solution containing 525 ppm of cesiumbicarbonate. The catalyst was dried and tested in a tube that is heatedby a salt bath. A gas mixture containing 15% ethylene, 7% oxygen, and78% inert, mainly nitrogen and carbon dioxide, was allowed to flow overthe catalyst under 300 p.s.i. The temperature of the reaction wasadjusted in order to obtain ethylene oxide productivity of 160 Kg perhour per m³ of catalyst. The results of the catalyst test are summarizedin table 5:

                  TABLE 5                                                         ______________________________________                                        Results of Catalyst Test                                                      Life, hr     Temp. °C.                                                                        Selectivity, %                                         ______________________________________                                        200          225       81.64                                                  ______________________________________                                    

EXAMPLE 7

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 5/16 inch cylindersThe support has a surface area of 0.55 m² /g pore volume of 0.3 cc/g,and medium pore diameter of 1.5μ. 95 parts of a cumene solution ofsilver neodecanoate, containing 26% silver, was added to 225 parts ofthe hot support and the mixture was mixed for 20 minutes. The depositionof silver was induced by heating the catalyst in a stream of nitrogen.

The catalyst was divided into several equal batches. Each batch wasimpregnated for two hours at room temperature in an ethanolic solutionthat contained a specific concentration of water and 525 ppm cesiumbicarbonate. The liquid was drained and followed by heating on a movingbelt in a current of 200° C. air.

A sample of the catalyst was tested in a tube that is heated by a saltbath. A gas mixture containing 15% ethylene, 7% oxygen, and 78% ofinert, mainly nitrogen and carbon dioxide, was allowed to flow over thecatalyst under 300 p.s.i. The temperature of the reaction was adjustedin order to obtain ethylene oxide productivity of 160 Kg per hour per m³of catalyst. The results of the catalyst test are summarized in table 6:

                  TABLE 6                                                         ______________________________________                                        Effect of Water in Cesium Solution                                                                  Performance*                                                                  at 150 Hr                                               Ex. 7         H.sub.2 O %                                                                           Sel (T °C.)                                      ______________________________________                                        A             <0.1    83.0 (231)                                              B             0.3     82.7 (230)                                              C             1       82.5 (232)                                              D             2       82.0 (228)                                              E             4       82.1 (225)                                              ______________________________________                                         *ΔEO = 1.5.                                                        

EXAMPLE 8

The method used to dry the solvent is of importance to the catalystperformance. After impregnation with cesium solution, it is ratherimportant to deposit the cesium salt on the catalyst as fast aspossible. It was discovered that drying the solvent in a current of hotgas, example heated air, is one of the efficient ways to achieve thefast deposition of the salt. The temperature of the gas has to be highenough to insure the fast precipitation of the cesium salt. Drying thecatalyst at a slow rate, as in vacuum or via a current of a lowtemperature gas, gives a catalyst with poor cesium dispersion and doesnot lead to the full benefit of the effect of the promoter. Thefollowing examples will illustrate the impact of the drying method:

A large batch of catalysts was prepared by impregnating a commercialalpha-alumina support with a solution of silver neodecanoate in cumene,followed by calcining the catalyst via heating at 500° C. in a stream ofnitrogen. The batch was divided into 245 g portions and each portion wasimpregnated for two hours with 300 g of ethanol solution that contain525 ppm cesium. The wet catalysts were dried using different methods.Table 7 illustrates the effect of the different cesium salts and thedrying method on the catalyst's performance:

                  TABLE 7                                                         ______________________________________                                        The effect of anhydrous Cesium solution                                       and the drying method on Selectivity                                          Temp. °C.                                                                              Sel. %                                                        Example 8                                                                             100 hr  200 hr  100 hr                                                                              200 hr                                                                              Notes                                     ______________________________________                                        A       233     232     81.8  82.3  Anhydrous                                                                     Cs.sub.2 CO.sub.3,                                                            Dried fast on belt                        B       236     237     80.7  80.9  CsOH in H2O/                                                                  ethanol, dried by                                                             vacuum for 17 hrs                         C       231     230     80.8  81.0  CsOH in H2O/                                                                  ethanol, dried on                                                             belt, 500° C.                      ______________________________________                                         ΔEO = 1.5                                                               SV = 5500                                                                

Example 8A illustrates the effect of both anhydrous cesium salt and thefast drying method. Example 8B is the standard case in which the cesiumsolution was not anhydrous and the catalyst was dried via maintainingits temperature below 50° C. under reduced pressure, 100 mm Hg, for 17hours. Example 8C is similar to 8B except a fast drying method was used.

EXAMPLE 9

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 5/16 inch cylindersThe support has a surface area of 0.55 m² /g pore volume of 0.3 cc/g,and medium pore diameter of 1.5μ. 95 parts of a cumene solution ofsilver neodecanoate, containing 26% silver, was added to 225 parts ofthe hot support and the mixture was mixed for 30 minutes. The depositionof silver was induced by heating the catalyst to a temperature that didnot exceed 500° C. in a stream of nitrogen. The residence time of thecatalyst in the heated zone was two minutes.

The catalyst was then impregnated for two hours at room temperature inan anhydrous ethanolic solution containing 525 ppm of cesium chloride.The solution was drained and an equal volume of ethanol was added. Thecatalyst was mixed with the fresh ethanol to remove any excess cesiumpresent on its surface. The liquid was drained, and the catalyst wasdried by placing it on a belt that travelled through a heated zone of afurnace in a current of 200° C. air. The residence time in the hot zone,the air flow and the furnace temperature were sufficient to dry all thesolvent in the shortest time possible.

After drying the catalyst, it was tested in a tube that is heated by asalt bath. A gas mixture containing 15% ethylene, 7% oxygen, and 78%inert, mainly nitrogen and carbon dioxide, was allowed to flow over thecatalyst under 300 p.s.i. The temperature of the reaction was adjustedin order to obtain ethylene oxide productivity of 160 Kg per hour per m³of catalyst. The results of the catalyst test are summarized in table 4:

                  TABLE 8                                                         ______________________________________                                        Time in Reactor                                                                             Selectivity                                                                             Reaction Temperature                                  Hr            %         °C.                                            ______________________________________                                        140           82.2      233                                                   ______________________________________                                    

EXAMPLE 10

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 5/16 inch cylindersThe support has a surface area of 0.55 m² /g pore volume of 0.3 cc/g,and medium pore diameter of 1.5μ. 95 parts of a cumene solution ofsilver neodecanoate, containing 26% silver, was added to 225 parts ofthe hot support and the mixture was mixed for 30 minutes. The depositionof silver was induced by heating the catalyst to a temperature that didnot exceed 200° C. in a stream of nitrogen. The residence time of thecatalyst in the heated zone was two minutes. This process was repeatedat 300° C. and 400° C.

The catalyst was then impregnated for two hours at room temperature inan anhydrous ethanolic solution containing 525 ppm of cesium carbonate.The liquid was drained, and the catalyst was dried by placing it on abelt that travelled through a heated zone of a furnace in a current of200° C. air. The residence time in the hot zone, the air flow and thefurnace temperature were sufficient to dry all the solvent in theshortest time possible.

After drying the catalyst, it was tested in a tube that is heated by asalt bath. A gas mixture containing 15% ethylene, 7% oxygen, and 78%inert, mainly nitrogen and carbon dioxide, was allowed to flow over thecatalyst under 300 p.s.i. The temperature of the reaction was adjustedin order to obtain ethylene oxide productivity of 160 Kg per hour per m³of catalyst. The results of the catalyst test are summarized in table 5:

                  TABLE 9                                                         ______________________________________                                        Time in Reactor                                                                             Selectivity                                                                             Reaction Temperature                                  Hr            %         °C.                                            ______________________________________                                        100           82.5      231                                                   300           82.7      230                                                   450           83.0      230                                                   ______________________________________                                    

EXAMPLE 11

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 5/16 inch cylinders.The support has a surface area of 0.55 m² /g, pore volume of 0.3 cc/g,and medium pore diameter of 1.5μ. 95 parts of a cumene solution ofsilver neodecanoate, containing 26% silver, was added to 225 parts ofthe hot support and the mixture was mixed for 30 minutes.

The deposition of silver was induced by heating the catalyst up to thedecomposition temperature of the silver salt. This was achieved viaheating in a furnace that has several heating zones in a controlledatmosphere. The catalyst was loaded on a moving belt that entered thefurnace at ambient temperature, which was gradually increased as thecatalyst passed from one zone to the next. The temperature was increase,up to 500° C., as the catalyst passed through seven heating zones. Afterthe heating zones the belt passed through a cooling zone that graduallycooled the catalyst to a temperature lower than 100° C. The totalresidence time in the furnace was 22 minutes. The atmosphere of thefurnace was controlled to eliminate uncontrolled combution of theorganic portion of the silver salt and solvent. This was achieved viausing nitrogen flow in the different heating zones. The amount ofnitrogen was sufficient to inhibit any combustion and to remove anyevolved fumes during the calcination process.

The catalyst was then impregenated for two hours at room temperature inan ethanolic solution of cesium bicarbonate. The catalyst was dried andtested in a tube that is heated by a salt bath. A gas mixture containing15% ethylene, 7% oxygen and 78% inert, mainly nitrogen and carbondioxide, was allowed to flow over the catalyst under 300 p.s.i. Thetemperature of the reaction was adjusted in order to obtain ethyleneoxide productivity of 160 Kg per hour per m3 of catalyst. The results ofthe catalyst test are summarized in Table 10 below:

                  TABLE 10                                                        ______________________________________                                        Catalyst life, hr                                                                           Temp. °C.                                                                        Selectivity, %                                        ______________________________________                                        200           229       82.55                                                 ______________________________________                                    

The invention claimed is:
 1. In the process for preparing a supportedsilver catalyst for the vapor-phase oxidation of ethylene to ethyleneoxide, comprising the steps of:(a) impregnating a porous support havinga surface area of about 0.2 to 2.0 m^(2/) g with a hydrocarbon solutionof a silver salt of an organic acid which is substantially free of waterand acid and sufficient to provide 3 to 25 wt % silver on the support;and (b) subjecting the silver impregnated support of step (a) toactivation by heating, (c) impregnating the activated silver impregnatedsupport of step (b) with solution containing an alkali metal compound toobtain a finished catalyst having about 1 to 6×10⁻³ gew of the alkalimetal per kg of catalyst;wherein the improvement is that said alkalicontaining solution is a substantially anhydrous alcoholic solutioncontaining cesium carbonate or cesium bicarbonate.
 2. The processaccording to claim 1 wherein said silver salt is the silver salt of aneo acid.
 3. The process according to claim 2 wherein said alkalisolution is anhydrous.
 4. The process according to claim 2 whereincesium carbonate is present.
 5. The process according to claim 2 whereincesium bicarbonate is present.
 6. The process according to claim 1wherein cesium carbonate is present.
 7. The process according to claim 1wherein cesium bicarbonate is present.